From: Alex Williamson , Christoph Lameter When using a time interpolator that is susceptible to jitter there's potentially contention over a cmpxchg used to prevent time from going backwards. This is unnecessary when the caller holds the xtime write seqlock as all readers will be blocked from returning until the write is complete. We can therefore allow writers to insert a new value and exit rather than fight with CPUs who only hold a reader lock. Signed-off-by: Alex Williamson Signed-off-by: Christoph Lameter Signed-off-by: Andrew Morton --- kernel/timer.c | 17 +++++++++++++---- 1 files changed, 13 insertions(+), 4 deletions(-) diff -puN kernel/timer.c~optimize-writer-path-in-time_interpolator_get_counter kernel/timer.c --- devel/kernel/timer.c~optimize-writer-path-in-time_interpolator_get_counter 2005-08-01 17:39:10.000000000 -0700 +++ devel-akpm/kernel/timer.c 2005-08-01 17:39:10.000000000 -0700 @@ -1429,7 +1429,7 @@ static inline u64 time_interpolator_get_ } } -static inline u64 time_interpolator_get_counter(void) +static inline u64 time_interpolator_get_counter(int writelock) { unsigned int src = time_interpolator->source; @@ -1443,6 +1443,15 @@ static inline u64 time_interpolator_get_ now = time_interpolator_get_cycles(src); if (lcycle && time_after(lcycle, now)) return lcycle; + + /* When holding the xtime write lock, there's no need + * to add the overhead of the cmpxchg. Readers are + * force to retry until the write lock is released. + */ + if (writelock) { + time_interpolator->last_cycle = now; + return now; + } /* Keep track of the last timer value returned. The use of cmpxchg here * will cause contention in an SMP environment. */ @@ -1456,7 +1465,7 @@ static inline u64 time_interpolator_get_ void time_interpolator_reset(void) { time_interpolator->offset = 0; - time_interpolator->last_counter = time_interpolator_get_counter(); + time_interpolator->last_counter = time_interpolator_get_counter(1); } #define GET_TI_NSECS(count,i) (((((count) - i->last_counter) & (i)->mask) * (i)->nsec_per_cyc) >> (i)->shift) @@ -1468,7 +1477,7 @@ unsigned long time_interpolator_get_offs return 0; return time_interpolator->offset + - GET_TI_NSECS(time_interpolator_get_counter(), time_interpolator); + GET_TI_NSECS(time_interpolator_get_counter(0), time_interpolator); } #define INTERPOLATOR_ADJUST 65536 @@ -1491,7 +1500,7 @@ static void time_interpolator_update(lon * and the tuning logic insures that. */ - counter = time_interpolator_get_counter(); + counter = time_interpolator_get_counter(1); offset = time_interpolator->offset + GET_TI_NSECS(counter, time_interpolator); if (delta_nsec < 0 || (unsigned long) delta_nsec < offset) _